In manufacturing semiconductor devices, small features or small geometric patterns are created by using optical photolithography. Typically, optical photolithography is achieved by projecting or by transmitting light through a pattern made of optically opaque areas and optically clear areas on a mask. The optically opaque areas of the pattern block the light, thereby casting shadows and creating dark areas, while the optically clear areas allow the light to pass, thereby creating light areas. Once the light areas and dark areas are formed, they are projected onto and through a lens and subsequently onto a photosensitive layer (e.g., resist) on a semiconductor wafer. Typically, the lens reduces the dimensions of the light and dark areas or pattern by a predetermined amount. Projecting light areas and dark areas on the resist results in portions of the resist being exposed, while other portions of the resist will be unexposed.
After exposure, the resist is developed to remove either the exposed portions of resist for a positive tone resist or the unexposed portions of resist for a negative tone resist. The patterned resist can then be used during a subsequent semiconductor fabrication process such as ion implantation or etching.
As microcircuit densities have increased, the size of the features of semiconductor devices have decreased to the sub-micron level. These sub-micron features may include the width and spacing of metal conducting lines or the size of various geometric features of active semiconductor devices. The requirement of sub-micron features in semiconductor manufacture has necessitated the development of improved lithographic processes and systems. One such improved lithographic process is known as phase-shift lithography.
With phase-shift lithography the interference of light energy is used to overcome diffraction and improve the resolution and depth of optical images projected onto a target. In phase-shift lithography, the phase of an exposure light at the object is controlled such that adjacent bright areas are formed preferably 180 degrees out of phase with one another. Dark regions are thus produced between the bright areas by destructive interference even when diffraction would otherwise cause these areas to be illuminated. This technique improves total resolution at the object.
In general, a phase-shifting photomask is constructed with a repetitive pattern formed of three distinct layers or areas. An opaque layer provides areas that allow no light transmission, a transparent layer provides areas which allow close to 100% of light to pass through, and a phase-shift layer provides areas which allow close to 100% of light to pass through but phase-shifted 180 degrees from the light passing through the transparent areas. The transparent areas and phase-shift areas are situated such that light energy diffracted through each area are canceled out in a darkened area therebetween. This creates the pattern of dark and bright areas which can be used to clearly delineate features of a pattern defined by the opaque layer of the mask on a photo patterned semiconductor wafer.
Another type of phase-shifting photo mask used in chromeless phase-shifting lithography (CPL) is known in the art as a chromeless phase-shifting mask (CPM). A CPM has no opaque (e.g., chrome) areas. Rather, the edges between the phase-shift areas and light transmission areas on the mask form a pattern of dark lines on the wafer. A CPM includes a transparent substrate with a raised or recessed phase-shifting area. The phase-shifting area may be formed by an additive or a subtractive process. The phase-shift can be created, for example, by etching a quartz substrate of the mask to a depth that is dependent on the wavelength of the imaging system.
Generally, with light being thought of as a wave, phase-shifting with a CPM is achieved by effecting a change in timing or by effecting a shift in waveform of a regular sinusoidal pattern of light waves that propagate through a transparent material. Typically, phase-shifting is achieved by passing light through areas of a transparent material of either differing thicknesses or through materials with different refractive indexes, thereby changing the phase or the period pattern of the light wave.
CPMs reduce diffraction effects by combining both phase-shifted light and non-phase-shifted light so that constructive and destructive interference takes place. Generally, a summation of constructive and destructive interference of phase-shift masks results in improved resolution and in improved depth of focus of a projected image of an optical system. Additionally, there is no need for a second exposure of a trim mask to remove unwanted phase edges, thereby simplifying the manufacturing process.
Referring to FIG. 1, a prior art CPM 10 for patterning contact holes is shown. The CPM 10 includes a repeating pattern of square shape features 12 formed on phase-shifted glass. The dimensions of the square shape features 12 are proportional to the dimensions of the contact holes to be patterned. The square shape features 12 have a length 14 and width 16, both of which are defined as having a dimension of “C”. Separating each square shape feature 12 are vertical strips 18 and horizontal strips 20, each having a width 22, wherein the width has a dimension of “S”. The vertical and horizontal strips 18, 20 define the boundaries of each square shape feature 12. Additionally, the vertical and horizontal strips form edges between phase-shift areas and light transmission areas on the CPM and, therefore, form a pattern of dark lines on a target device when exposed to light energy.
With additional reference to FIG. 2A, an exemplary image 30 obtained using the CPM 10 to expose a photosensitive layer to light energy is shown. The CPM 10 has a “C” dimension of about 160 nanometers (nm), an “S” dimension of about 40 nm, and a pitch of about 200 nm (i.e., the separation between center lines of adjacent contact holes). The image 30 formed in the photosensitive layer includes a pattern of contact holes 32, which, as noted above, are proportional to the length 14 and width 16 of the square shape features 12 on the CPM 10. Thus, the dimensions of the contact holes 32 can be varied by varying the length 14 and width 16 of the square shape pattern 12. The width of the vertical and horizontal strips 18, 20 determine the separation between adjacent contact holes.
As the length 14 and width 16 of each square shape pattern 12 and the width 22 of the strips 18, 20 decrease (i.e., “C” and “S” are decreased), the dimensions of each resulting contact hole image as well as the separation between adjacent contact hole images also decrease. As the values of “C” and/or “S” are reduced below a particular threshold, however, the contact pattern projected on the photosensitive layer becomes distorted or fails to image at all. This distortion is due to optical interference or lack thereof generated by light energy passing through adjacent square shape features 12. At larger feature sizes (e.g., “C” and “S” above a certain threshold), the interference is insignificant. As the feature size is reduced, however, the interference becomes significant and the pattern does not image as desired.
Referring to FIG. 2B, an image 40 is illustrated that was obtained using a CPM 10 with a “C” dimension of about 150 nm, an “S” dimension of about 50 nm, and a pitch of about 200 nm. The resulting image 40 includes contact hole patterns 32′, along with dark spots 42 and intermediately bright spots 44. As should be appreciated, the dark spots 42 and the intermediately bright spots 44 are not desirable. FIG. 2C shows an image 40′ obtained using a CPM 10 with a “C” dimension of 140 nm, an “S” dimension of 60 nm, and a pitch of 200 nm. In the second image 40′ of FIG. 2C, the contact holes no longer image. Instead, an image is obtained that includes numerous dark spots 42′ and bright spots 46 of varying dimensions. Again, the image 40′ is not desirable.
Presently, chromeless phase-shift mask technology can accurately image patterns, such as contact holes, down to about 100 nm. A pervasive trend in modern integrated circuit manufacture is to produce semiconductor devices that are as small as possible. As this trend continues, CPM technology will soon reach a limitation where it can no longer pattern images required for modern integrated circuits.
Accordingly, there is a need in the art for a device and method of patterning sub-100 nm contact holes using CPL.